–You're considering only variable costs, my friend. There are fixed costs too: store's rent (or the opportunity cost if you own), insurance, etc. Anyways, thanks for the entertainment and going into the minutiae of burger selling. I could have said "$300 airfare" or "$0.75 pen", etc.
This is a very interesting topic.
I am by no means an econ guru. I am in manufacturing/sourcing. Yes those are variable costs. But if you exclude technology, the US manufacturing asset base is relatively depreciated making variable costs more of the driver. I see this every day in the chemical industry. 10 or 20 (or 30) years ago it made more sense to build a new plant and depreciate over 10-20 years. Today I see more "fixing" which is fully expensed.
